A cellulose acylate film has historically been used as an optical material and a substrate of photographic materials because of its toughness and flame retardancy. In particular, a cellulose acylate film has recently found frequent use as a transparent optical film for LCD application. Because a cellulose acylate film exhibits high optical transparency and isotropy, it is an excellent optical material for use in equipment dealing with polarization such as LCDs. Accordingly, it has been used as a polarizer protective film or a substrate of an optical compensation film that can improve display quality when viewed at an oblique angle (viewing angle compensation).
A polarizing plate used in an LCD is composed of a polarizer and a protective film on at least one side of the polarizer. An ordinary polarizer is obtained by staining a stretched polyvinyl alcohol (PVA) film with iodine or a dichroic dye. It is important for a protective film for the polarizer to have high optical isotropy. The characteristics of a polarizer are largely governed by the optical characteristics of the polarizer protective film. Therefore, a cellulose acylate film, particularly a triacetyl cellulose film, has often been used as a polarizer protective film.
The demand for LCDs with improved viewing angle characteristics has ever been increasing thereby boosting the demand for a polarizer protective film with higher optical isotropy. In other words, a protective film for a polarizer is keenly required to have not only a smaller front retardation (in-plane retardation) (hereinafter, “Re”) but a smaller thickness direction retardation (hereinafter “Rth”).
JP-A 2002-249599 and JP-A 2001-247717 propose adding a certain low-molecular compound to a cellulose acylate to improve the optical isotropy. The techniques disclosed necessitate the compound as an additive chosen from those having small volatility and good compatibility with the cellulose acylate. Further, when the production conditions of a film are regulated for the purpose of improving the uniformity of the optical characteristic distribution, there was a case where Rth sometimes could not be lowered even by using the foregoing material. Besides, although optical isotropy is achieved in a specific wavelength region, the isotropy cannot be seen as sufficient over the whole visible region, leaving room for further improvement.
An additional problem of the above techniques is that the film obtained has a high equilibrium water content. A polarizing plate having such a film as a protective film undergoes reduction in polarizing performance when used under a high temperature and humidity condition, still needing improvement.